Question

Which of the following statements are correct A. A negative feedback can increase linearity and reduce distortion. B. Positive feedback always improves stability of the amplifier. C. Series-shunt (voltage) negative feedback tends to reduce output impedance. D. Closed-loop bandwidth typically increases when negative feedback is applied. Choose the correct answer from the options given below :

• A. A, B, C only
• B. A, C, D only
• C. A and C only
• D. A and D only

Hint:

Negative feedback generally:
• Improves stability
• Reduces distortion
• Increases bandwidth
• Improves linearity Positive feedback generally reduces stability and may cause oscillations.

Answer 1

The Correct Option is B

Concept: Feedback is one of the most important concepts in amplifier design. There are two major types of feedback:
• Negative feedback
• Positive feedback Negative feedback improves amplifier performance in several ways:
• Reduces distortion
• Improves linearity
• Increases bandwidth
• Stabilizes gain
• Can reduce output impedance depending on topology Positive feedback generally:
• Increases gain instability
• Can lead to oscillations
• Is used in oscillators and regenerative circuits Thus, careful conceptual analysis is required for each statement.

Step 1: Analyze Statement A. Statement A says: Negative feedback can increase linearity and reduce distortion. This statement is correct. When negative feedback is applied:
• Nonlinearities of the amplifier are reduced.
• Harmonic distortion decreases.
• Gain becomes more stable and linear. Hence: \[ A \text{ is correct} \]

Step 2: Analyze Statement B. Statement B says: Positive feedback always improves stability of the amplifier. This statement is incorrect. In reality:
• Positive feedback tends to decrease stability.
• Excessive positive feedback can produce oscillations.
• Amplifier output may become uncontrollable. Positive feedback is intentionally used in:
• Oscillators
• Schmitt triggers
• Regenerative circuits Therefore: \[ B \text{ is incorrect} \]

Step 3: Analyze Statement C. Statement C says: Series-shunt (voltage) negative feedback tends to reduce output impedance. This statement is correct. In voltage-series feedback:
• Output voltage is sampled.
• Feedback is applied in series at the input. This topology:
• Increases input impedance
• Reduces output impedance Reduced output impedance improves voltage regulation and load-driving capability. Hence: \[ C \text{ is correct} \]

Step 4: Analyze Statement D. Statement D says: Closed-loop bandwidth typically increases when negative feedback is applied. This statement is also correct. Negative feedback causes:
• Gain reduction
• Bandwidth expansion The gain-bandwidth product remains approximately constant: \[ A_v \times BW \approx \text{constant} \] Thus, when gain decreases due to feedback: \[ BW \uparrow \] Hence: \[ D \text{ is correct} \]

Step 5: Determine the correct combination. Correct statements are: \[ A,\ C,\ D \] Incorrect statement: \[ B \] Therefore, the correct option is: \[ \boxed{(B)\ A,C,D\text{ only}} \]